Under high load it was possible that memory mapping was started on a table
while other threads were working with the table.

I fixed the start of memory mapping so that it is done at the first table
open or when the requesting thread is using the table exclusively only.


include/my_base.h:
  Bug#25460 - High concurrency MyISAM access causes severe mysqld crash.
  Added a new MyISAM open_flag HA_OPEN_MMAP.
storage/myisam/ha_myisam.cc:
  Bug#25460 - High concurrency MyISAM access causes severe mysqld crash.
  Replaced the call to mi_extra(... HA_EXTRA_MMAP ...) by the new open_flag
  HA_OPEN_MMAP. This effects that the mapping will no longer be done on
  every open of the table but just on the initial open, when the MyISAM
  share is created.
storage/myisam/mi_dynrec.c:
  Bug#25460 - High concurrency MyISAM access causes severe mysqld crash.
  Added a comment with a concern regarding the mmap flag MAP_NORESERVE.
storage/myisam/mi_extra.c:
  Bug#25460 - High concurrency MyISAM access causes severe mysqld crash.
  Limited the start of memory mapping to situations where the requesting
  thread has the table exclusively opened.
storage/myisam/mi_open.c:
  Bug#25460 - High concurrency MyISAM access causes severe mysqld crash.
  Added memory mapping code. Used if the new open_flag HA_OPEN_MMAP is set.